[Xamarin.Android.Build.Tasks] Add $(AndroidLinkResources) (#5317)

The generated `Resource` class in `Resource.designer.cs` can be quite
large.

When doing "Release" builds, update the linker to *remove* the
`Resource` class and its nested types.  We do this by examining all
the locations where the fields are used and replacing those IL calls
with a call which specifies the actual resource id value directly.

This will remove the need to call
`global::Android.Runtime.ResourceIdManager.UpdateIdValues();` from
the static constructors in the `Resource` class and its nested types,
which will improve app launch times (as `UpdateIdValues()` involves
Reflection, which is not "fast").

Due to an odd linker failure we cannot completely remove the
`Resource` class itself.  It *will* be completely empty.

"Debug" builds will *not* remove the `Resource` type, and will retain
the call to `UpdateIdValues()`, in order to ensure that app rebuild
and redeploy times are as fast as possible.

The new `$(AndroidLinkResources)` MSBuild property has been added to
control this feature.  It is disabled by default for now.

`$(AndroidLinkResources)`=True appears to save between 100-280kb for
an `.apk`.  This is dependent on how many assemblies are Android
assemblies and contain or use Resources.

Xamarin.Forms base app `apkdiff` results

	Size difference in bytes ([*1] apk1 only, [*2] apk2 only):
	  -          10 assemblies/Mono.Android.dll
	  -       4,001 assemblies/Xamarin.Essentials.dll
	  -      45,426 assemblies/Xamarin.Forms.Platform.Android.dll
	  -     102,839 assemblies/formstest.Android.dll
	Summary:
	  +           0 Other entries 0.00% (of 887,808)
	  +           0 Dalvik executables 0.00% (of 3,341,152)
	  -     152,276 Assemblies -3.23% (of 4,709,077)
	  +           0 Shared libraries 0.00% (of 41,693,796)
	  -     151,552 Package size difference -0.73% (of 20,820,695)

Maps sample `apkdiff` results

	Size difference in bytes ([*1] apk1 only, [*2] apk2 only):
	  -           4 lib/armeabi-v7a/libxamarin-app.so
	  -          14 assemblies/Mono.Android.dll
	  -       4,757 assemblies/Xamarin.Essentials.dll
	  -      46,051 assemblies/Xamarin.Forms.Platform.dll
	  -      47,586 assemblies/Xamarin.Forms.Platform.Android.dll
	  -      48,470 assemblies/Xamarin.Forms.Maps.Android.dll
	  -     135,589 assemblies/FormsMapsSample.Android.dll
	Summary:
	  +           0 Other entries 0.00% (of 1,137,209)
	  +           0 Dalvik executables 0.00% (of 2,519,972)
	  -     282,467 Assemblies -5.51% (of 5,123,833)
	  -           4 Shared libraries -0.00% (of 41,843,596)
	  -     282,624 Package size difference -1.34% (of 21,075,664)

Author: dellis1972

Documentation/guides/building-apps/build-properties.md

@@ -696,6 +696,27 @@ used in Android Application projects. The default value is
 <AndroidLinkMode>SdkOnly</AndroidLinkMode>
 ```
 
+## AndroidLinkResources
+
+When `true` this will make the build system link out the Nested Types
+of the Resource.Designer.cs `Resource` class in all assemblies. The
+IL code that uses those types will be updated to use the values
+directly rather than accessing fields.
+
+This can have a small impact on reducing the apk size, it might also
+help slightly with startup performance. This will only effect "Release"
+based builds.
+
+***Experimental***.  Only designed to work with code such as
+
+```
+var view = FindViewById(Resources.Ids.foo);
+```
+
+Any other scenarios (such as reflection) will not be supported.
+
+Added in Xamarin.Android 11.3.
+
 ## AndroidLinkSkip
 
 Specifies a semicolon-delimited (`;`)

src/Microsoft.Android.Sdk.ILLink/SetupStep.cs

@@ -65,6 +65,11 @@ protected override void Process ()
 			if (Context.TryGetCustomData ("AddKeepAlivesStep", out addKeepAlivesStep) && bool.TryParse (addKeepAlivesStep, out var bv) && bv)
 				InsertAfter (new AddKeepAlivesStep (cache), "CleanStep");
 
+			string androidLinkResources;
+			if (Context.TryGetCustomData ("AndroidLinkResources", out androidLinkResources) && bool.TryParse (androidLinkResources, out var linkResources) && linkResources) {
+				InsertAfter (new RemoveResourceDesignerStep (),  "CleanStep");
+				InsertAfter (new GetAssembliesStep (), "CleanStep");
+			}
 			InsertAfter (new StripEmbeddedLibraries (),  "CleanStep");
 		}
 

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/AndroidLinkConfiguration.cs

@@ -0,0 +1,27 @@
+using Mono.Cecil;
+using Mono.Linker;
+using Mono.Linker.Steps;
+using System;
+using System.Linq;
+using Xamarin.Android.Tasks;
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using Mono.Cecil.Cil;
+
+namespace MonoDroid.Tuner
+{
+	public class AndroidLinkConfiguration {
+		public List<AssemblyDefinition> Assemblies { get; private set; } = new List<AssemblyDefinition> ();
+
+		static ConditionalWeakTable<LinkContext, AndroidLinkConfiguration> configurations = new ConditionalWeakTable<LinkContext, AndroidLinkConfiguration> ();
+
+		public static AndroidLinkConfiguration GetInstance (LinkContext context)
+		{
+			if (!configurations.TryGetValue (context, out AndroidLinkConfiguration config)) {
+				config = new AndroidLinkConfiguration ();
+				configurations.Add (context, config);
+			}
+			return config;
+		}
+	}
+}

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/Extensions.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 
 using Mono.Cecil;
+using Mono.Cecil.Cil;
 
 using Mono.Linker;
 
@@ -327,5 +328,39 @@ public static bool TryGetMarshalMethod (this MethodDefinition method, string nat
 
 			return false;
 		}
+
+		public static Instruction CreateLoadArraySizeOrOffsetInstruction (int intValue)
+		{
+			if (intValue < 0)
+				throw new ArgumentException ($"{nameof (intValue)} cannot be negative");
+
+			if (intValue < 9) {
+				switch (intValue) {
+				case 0:
+					return Instruction.Create (OpCodes.Ldc_I4_0);
+				case 1:
+					return Instruction.Create (OpCodes.Ldc_I4_1);
+				case 2:
+					return Instruction.Create (OpCodes.Ldc_I4_2);
+				case 3:
+					return Instruction.Create (OpCodes.Ldc_I4_3);
+				case 4:
+					return Instruction.Create (OpCodes.Ldc_I4_4);
+				case 5:
+					return Instruction.Create (OpCodes.Ldc_I4_5);
+				case 6:
+					return Instruction.Create (OpCodes.Ldc_I4_6);
+				case 7:
+					return Instruction.Create (OpCodes.Ldc_I4_7);
+				case 8:
+					return Instruction.Create (OpCodes.Ldc_I4_8);
+				}
+			}
+
+			if (intValue < 128)
+				return Instruction.Create (OpCodes.Ldc_I4_S, (sbyte)intValue);
+
+			return Instruction.Create (OpCodes.Ldc_I4, intValue);
+		}
 	}
 }

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/GetAssembliesStep.cs

@@ -0,0 +1,28 @@
+using Mono.Cecil;
+using Mono.Linker;
+using Mono.Linker.Steps;
+using System;
+using System.Linq;
+using Xamarin.Android.Tasks;
+using System.Collections.Generic;
+using Mono.Cecil.Cil;
+
+namespace MonoDroid.Tuner
+{
+	public class GetAssembliesStep : BaseStep
+	{
+		AndroidLinkConfiguration config = null;
+
+		protected override void Process ()
+		{
+			config = AndroidLinkConfiguration.GetInstance (Context);
+		}
+
+		protected override void ProcessAssembly (AssemblyDefinition assembly)
+		{
+			if (config == null)
+				return;
+			config.Assemblies.Add (assembly);
+		}
+	}
+}

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/Linker.cs

@@ -77,7 +77,7 @@ static Pipeline CreatePipeline (LinkerOptions options)
 				pipeline.AppendStep (new LoadI18nAssemblies (options.I18nAssemblies));
 
 			pipeline.AppendStep (new BlacklistStep ());
-			
+
 			foreach (var desc in options.LinkDescriptions)
 				pipeline.AppendStep (new ResolveFromXmlStep (new XPathDocument (desc)));
 
@@ -121,6 +121,11 @@ static Pipeline CreatePipeline (LinkerOptions options)
 			pipeline.AppendStep (new StripEmbeddedLibraries ());
 			if (options.AddKeepAlives)
 				pipeline.AppendStep (new AddKeepAlivesStep (cache));
+
+			if (options.LinkResources) {
+				pipeline.AppendStep (new GetAssembliesStep ());
+				pipeline.AppendStep (new RemoveResourceDesignerStep ());
+			}
 			// end monodroid specific
 			pipeline.AppendStep (new RegenerateGuidStep ());
 			pipeline.AppendStep (new OutputStepWithTimestamps ());

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/LinkerOptions.cs

@@ -25,5 +25,6 @@ class LinkerOptions
 		public bool AddKeepAlives { get; set; }
 		public bool PreserveJniMarshalMethods { get; set; }
 		public bool DeterministicOutput { get; set; }
+		public bool LinkResources { get; set; }
 	}
 }

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/MonoDroidMarkStep.cs

@@ -188,7 +188,7 @@ void UpdateMagicPrefill (TypeDefinition magicType)
 			var instructions = methodPrefill.Body.Instructions;
 			instructions.Clear ();
 
-			instructions.Add (CreateLoadArraySizeOrOffsetInstruction (marshalTypes.Count));
+			instructions.Add (Extensions.CreateLoadArraySizeOrOffsetInstruction (marshalTypes.Count));
 			instructions.Add (Instruction.Create (OpCodes.Newobj, magicType.Module.ImportReference (genericMethodDictionaryCtor)));
 			instructions.Add (Instruction.Create (OpCodes.Stsfld, fieldTypesMap));
 
@@ -197,7 +197,7 @@ void UpdateMagicPrefill (TypeDefinition magicType)
 			foreach (var type in marshalTypes) {
 				instructions.Add (Instruction.Create (OpCodes.Ldsfld, fieldTypesMap));
 				instructions.Add (Instruction.Create (OpCodes.Ldstr, type.FullName.Replace ("/__<$>_jni_marshal_methods", "").Replace ("/","+")));
-				instructions.Add (CreateLoadArraySizeOrOffsetInstruction (idx++));
+				instructions.Add (Extensions.CreateLoadArraySizeOrOffsetInstruction (idx++));
 				instructions.Add (Instruction.Create (OpCodes.Callvirt, importedMethodSetItem));
 			}
 
@@ -265,40 +265,6 @@ static bool IsLdcI4 (Instruction instruction, out int intValue)
 			return true;
 		}
 
-		static Instruction CreateLoadArraySizeOrOffsetInstruction (int intValue)
-		{
-			if (intValue < 0)
-				throw new ArgumentException ($"{nameof (intValue)} cannot be negative");
-
-			if (intValue < 9) {
-				switch (intValue) {
-				case 0:
-					return Instruction.Create (OpCodes.Ldc_I4_0);
-				case 1:
-					return Instruction.Create (OpCodes.Ldc_I4_1);
-				case 2:
-					return Instruction.Create (OpCodes.Ldc_I4_2);
-				case 3:
-					return Instruction.Create (OpCodes.Ldc_I4_3);
-				case 4:
-					return Instruction.Create (OpCodes.Ldc_I4_4);
-				case 5:
-					return Instruction.Create (OpCodes.Ldc_I4_5);
-				case 6:
-					return Instruction.Create (OpCodes.Ldc_I4_6);
-				case 7:
-					return Instruction.Create (OpCodes.Ldc_I4_7);
-				case 8:
-					return Instruction.Create (OpCodes.Ldc_I4_8);
-				}
-			}
-
-			if (intValue < 128)
-				return Instruction.Create (OpCodes.Ldc_I4_S, (sbyte)intValue);
-
-			return Instruction.Create (OpCodes.Ldc_I4, intValue);
-		}
-
 		bool UpdateMarshalRegisterMethod (MethodDefinition method, HashSet<string> markedMethods)
 		{
 			var instructions = method.Body.Instructions;
@@ -310,7 +276,7 @@ bool UpdateMarshalRegisterMethod (MethodDefinition method, HashSet<string> marke
 				if (!arraySizeUpdated && idx + 1 < instructions.Count) {
 					int length;
 					if (IsLdcI4 (instructions [idx++], out length) && instructions [idx].OpCode == OpCodes.Newarr) {
-						instructions [idx - 1] = CreateLoadArraySizeOrOffsetInstruction (markedMethods.Count);
+						instructions [idx - 1] = Extensions.CreateLoadArraySizeOrOffsetInstruction (markedMethods.Count);
 						idx++;
 						arraySizeUpdated = true;
 						continue;
@@ -351,7 +317,7 @@ bool UpdateMarshalRegisterMethod (MethodDefinition method, HashSet<string> marke
 						continue;
 
 					if (markedMethods.Contains (mr.Name)) {
-						instructions [offsetIdx] = CreateLoadArraySizeOrOffsetInstruction (arrayOffset++);
+						instructions [offsetIdx] = Extensions.CreateLoadArraySizeOrOffsetInstruction (arrayOffset++);
 						continue;
 					}
 
@@ -436,7 +402,7 @@ protected override void DoAdditionalTypeProcessing (TypeDefinition type)
 			foreach (MethodReference method in type.Methods)
 				MarkMethod (method);
 		}
-		
+
 		private void PreserveRegisteredMethod (TypeDefinition type, string member)
 		{
 			var type_ptr = type;